The ability to communicate via satellite while in motion, though long desirable, has been an elusive goal. While some mobile satellite communication capability has been developed, such as the INMARSAT, Iridium, Teledesic and other systems, these are directed to low bandwidth communications that have generally been limited to certain types of emergency signaling (in the case of INMARSAT) or voice, in the case of Iridium, Teledesic and the like. Higher bandwidth communications, particularly of the type to support the sort of broadband data transmission that most computer users have grown accustomed to over the past decade, have proved even more elusive for those wishing to access this ability from a mobile platform. The capability to have high bandwidth data transmission not only where one wishes, but also when, has been complicated by the fact that most or all of the high throughput satellite communications systems require a great deal of lead time as a precondition to invoking them. Typically, one is required to communicate with a satellite communications (“satcom”) service provider many hours or even days in advance in order to permit the provider to make the necessary resource allocation arrangements.
This constrained state of affairs, and the various technical, economic and organizational factors that have produced it, have conspired to keep high bandwidth mobile satcom from being realized, despite a vast potential demand for it.
Other factors are also relevant when considering the design of effective satcom systems. Communications satellite systems have been configured in a variety of ways, each with its own level of complexity, and each having respective advantages and disadvantages. One method for conducting satellite communications is sometimes referred to as the “bent-pipe” method, in which a signal is sent from a fixed point on Earth, received by the satellite and amplified, then sent back down to a predetermined receiver. Decisions about routing and switching of communications traffic, essential to communications systems in general, are made on the ground, as is the execution of those decisions. Because the satellites used in carrying out the bent-pipe method lack on-board communications traffic processing, the method is typically limited to use within a single satellite communications beam.
Another method for satellite communication is the “hub” configuration. In this configuration, a series of terrestrial terminals and a single hub are located within a single beam. The hub acts as a two-step bent-pipe configuration, in which the uplink signal is routed from the satellite, which may be a geosynchronously orbiting (GEO) satellite, to an intermediate ground hub. The hub acts as a local control center to assign channels and other functions associated with the network management.
Conventional satellite communications systems, which may be referred to below as SATCOM systems, when using satellite in GEO orbits, have typically provided two types of services: a relay mode and a broadcast mode. In the relay mode, the GEO satellite relays a signal from one terrestrial terminal to another. When in the broadcast mode, the GEO satellite transmits a signal to a large number of terrestrial terminals. In the relay mode, which corresponds to the bent-pipe discussed above, a terrestrial terminal transmits a signal using an uplink frequency to the GEO satellite, which retransmits the signal to a second terrestrial terminal using a downlink frequency. When the transmission footprint of the GEO satellite on the earth's surface is large, the power density of the signal is correspondingly low. A signal having low power density demands that the receiving antenna be sufficiently large to achieve the requisite antenna gain to put the low density signal to use. Alternatively, use of smaller antennas requires the satellite to generate sufficient radiated power to supply a power density within the single wide area coverage beam sufficient for signal reception and usage by the smaller antennas.
Communications satellite system architectures for smaller antennas have involved a number of smaller spot beams, instead of a single wide area coverage beam, to cover the same geographical area. By decreasing the size of the spot beams while maintaining the same overall transmitted power, the power density within each spot beam enables the use of smaller terrestrial antennas.
Existing satellite communications systems use common Ku-band uplink and Ku-band downlink frequencies that are extensively populated and re-used. Furthermore, in existing satellite communications systems the satellites are closely spaced. This close spacing increases the likelihood of interference between their respective communications links and the need to reuse frequencies. Furthermore, closely spaced satellites using the Ku-band require terrestrial terminals to use a narrow beam, which in turn requires larger antennas and more accurate pointing systems. As a result, terrestrial terminals become larger and more expensive, while providing a lower throughput than may be desirable.
Some existing satellite communication systems may use a steerable spot beam, a satellite signal that is specially concentrated in power so that it will cover only a regional geographical area and the direction of which can be controlled. Because these beams have a smaller, more regional footprint than do wide area beams, they are not easily adapted to terrestrial terminals that are on-the-move.